In large industrial and commercial premises, transformer energization may cause large inrush current that can be 15 times the nominal current. This phenomenon requires safety margins placed on overcurrent protection device settings so as to allow the devices to clear these currents; this in turn makes it difficult to establish sensitive protection settings, leading to high incident energy levels from electrical faults often resulting in damaging arc flash events.
Arc flash is one of the major causes of personal injuries and fatalities in large industrial facilities. Required safety margins and the effects of inrush current also limit the size of the transformation connected to the distribution system.
Many patents describe invention of devices that can detect the occurrence of an arc flash. The arc flash can be detected with measurement of light, such as in U.S. Pat. No. 9,046,391, measurement of pressure inside the cabinet or by measuring the level of noise. While these approaches are appropriate to detect the presence of an arc flash, there is a need to eliminate the root cause (magnitude of current or the amount of time that current can be present) of an arc flash event. An arc flash event and its severity is due to some form of short circuit within equipment, when the short circuit is not cleared fast enough—typically due to an intentional delay in the protection system. This delay is set in the protection relay to take into account the current inrush during transformer energization and avoid unwanted tripping. The present invention proposes a novel approach to eliminate the inrush current and therefore allow more sensitive protection settings that can clear an electrical fault faster. This aids in limiting the severity of human injuries and damage to equipment.
When a power transformer is de-energized, a residual magnetic flux may remain in the core of the power transformer. It is generally well known that due to that residual magnetic flux, the uncontrolled energization of a transformer may cause inrush currents having several orders of magnitudes of the rated current value of the transformer. To avoid any unwanted tripping during this energization process, the protection setting must include some delay to let the inrush current taper off and disappear.
Over the years, techniques were developed to mitigate and/or reduce inrush current. A well-known technique to mitigate power transformer inrush current uses a circuit breaker (CB) equipped with pre-insertion resistors/closing resistors. Another current technique for mitigating inrush current uses smoothing inductors along with the CB. However, these two known techniques require the use of more complex CBs with additional components and have proved to add major costs for both installation and maintenance. Therefore, it is well known that these mechanical add-ons increase the frequency of maintenance operations and reduce overall reliability.
A paper entitled “Elimination of Transformer Inrush Currents by Controlled Switching—Part I and II” published in the IEEE transactions on power delivery, Vol. 16, No. 2 in April 2001, discloses a novel approach making use of controlled switching techniques. This paper describes a method for controlling the closing of a circuit breaker at a precise electrical angle calculated based on the magnitude and polarity of the residual magnetic flux of the transformer.
Another paper, entitled “Transformer controlled switching taking into account the core residual flux a real case study” and published in CIGRE 13-201 session 2002, discloses demonstrated field results of the implementation of above mentioned technique. The controlled switching using an independently-operated pole circuit breaker has proved to effectively eliminate the inrush current. This approach uses different electrical closing angles on each pole of the circuit breaker according to the calculated residual magnetic flux in the transformer core (delayed closing strategy). The residual magnetic flux of each transformer phase resulting from de-energization is calculated using the mathematical integral of the transformer voltage. When energizing the power transformer, the closing angle of the circuit breaker is adjusted in such a way that the prospective magnetic flux produced by the energization matches or equals the residual magnetic flux in that phase. The two other phases are closed n half cycles after the zero crossing voltage edge preceding the first phase to be closed.
CSD techniques have also been used for current inrush mitigation for capacitor banks switching, reactors and power lines. The same technique is proposed in this standard in order to eliminate inrush current in many standard commercial/industrial interconnections with the utility. FIG. 1 shows such an interconnection. The distribution system (1) feeds the commercial or industrial installation. A fuse (2), with a value imposed by the utility, often limits the size of transformation and is sized as small as possible to clear transformer inrush while still providing adequate short circuit protection. The circuit breaker (3) is the customer owned device that can isolate the installation for maintenance purposes or for safety reasons and acts to protect the customer owned equipment against damaging overcurrent. The main power transformer (4) reduces the voltage on the distribution network to a lower value to feed the loads of the facility (6) in the installation (5). Often times, even if the fault is detected, a severe arc flash (7) event will still occur because the clearing time of the protection device is not fast enough to mitigate the amount of energy available to the faulted location. The protection relay (8) detects overload, short-circuits, and other electrical faults. On such occurrence, it trips the circuit breaker (3).
The inrush current, shown in FIG. 2, in a commercial or industrial installation is primarily caused by the uncontrolled energization of the main power transformer (4). The presence of residual flux in the transformer's magnetic core resulting from its previous de-energization is responsible for the high inrush current when the energization of the power transformer is uncontrolled, causing transformer saturation. At the present time, due to this inrush current, the setting of protective relays (8) must take into account the high level of inrush current, consequently these devices are less sensitive to faults in the downstream installation (5). Since the protection is less sensitive, detection of faults is less effective at minimizing and mitigating arc flash events. Furthermore, since the inrush current can rise up to 15 times the nominal current, the power capacity and size of the transformation at an installation is limited due to the detrimental transformer energization effects on the distribution system.
An arc flash happens when electric current flows through the air between conductors. It releases intense light, heat, sound, and blast of arc that are produced by vaporized components of enclosure material such as copper, steel, or aluminum. Injuries resulting from arc flash events in electric power systems are among the most traumatic and costly safety hazards.
FIG. 4 shows a single line diagram used for protection coordination simulations. In this example, a utility system (18) feeds a utility bus (19). The customer bus (21) is connected to the utility bus through a fuse (20).
On the customer side, a transformer (24) changes the voltage level from 25000 V to 600 V. A circuit breaker (23) can be operated manually (opened) to isolate the customer load (25) or it can be tripped (opened) by the overcurrent relay (22).
As illustrated in FIG. 5, time vs current coordination graph, the customer overcurrent protection relay (28) must be set so as to overcome the worst-case transformer inrush (29). On the same figure, the relay allows 180 A of continuous current flow. Since no CSD is used, the inrush current can be very high (up to 15 times the transformer full load amps). For this reason, the instantaneous portion of the overcurrent protection relay (28) is set at 1400 A.
FIG. 7 shows a single line diagram used for protection coordination simulations with typical protective relaying settings. With these settings and the layout shown on FIG. 7, the fault clearing time (FCT) would be up to 20 cycles (precisely FCT=20.339 cycles), which causes an incident energy (IE) level (potential severity of an arc flash event) of 52 cal/cm2 (precisely IE=52.40 cal/cm2 at 18″). The fault current (Ibf″) at the location would be 58.32 kA. In this example, the same elements as used in figure FIG. 4 were utilized. The utility system (34), utility bus (35), customer bus (36), transformer (38), overcurrent protection relay (37) and customer load (39). This kind of energy can be lethal for people working in the vicinity. It should be noted that working on levels of 40 calories per square centimeter or greater should be avoided at all times due to the blast hazards caused by an arc flash event at this level. FIG. 5 illustrates the time vs. current coordination graph for the aforementioned scenario showing the transformer thermal limits (27) and the fuse characteristics (26).
FIG. 9 shows the protection coordination curves of the FIG. 4 example but with a 8 MVA transformer (49) instead of 5 MVA. In this example, the overcurrent protection relay (50) still allows 180 A of continuous current, but now the instantaneous portion of the overcurrent protection relay (50) is set at 2280 A to overcome the larger inrush current (51) of the larger transformer. FIG. 9 shows that the overcurrent protection relay (50) setting does not provide sufficient clearance between itself and the utility fuse (48), and as such, this larger transformer connection would not typically be accepted on the utility.
There is thus a need for a new technique to reduce significantly the arc flash incident energy in commercial and industrial electrical installations that are connected to typical electric distribution networks by limiting transformer inrush current and allowing more sensitive protection settings.
There is also a need for a new technique to increase the energy capacity of electric installations.